Ferrite t circulator for coupling an antenna to a transmitter-receiver



Nov. 30, 1965 RABowsKY ETAL 3,221,255-

FERRITE T CIRCULAI'OR FOR COUPLING AN ANTENNA TO A TRANSMITTER-RECEIVER Filed oct. 1e, 1961 INPUT ARM 4 4445 426 H a 32 253 A 45 [8 57 58 2 2 FERRVT'E OUTPUT ARM l//N RABO W.S/ Y A/o'QMA/v F. WE/N//oa/.sf

INVENToRs 3,221,255 FERRITE T QIRCULA'EOR FR CGUPLING AN ANTENNA T A TRANSMITTER-RECEIVER Irving Rabowsiry, North Hollywood, and Norman P. Weinhouse, Woodland Hiils, Calif., assignors, by mesne `assignrnents, to Micro-Radionics, lne., Van Nuys,

alii.

fitted 0st. 16, 1961, Ser. No. 145,111 6 Claims. (Ci. S25-4) This invention relates to microwave systems, and more particularly to a coupling arrangement for improving the coaction between an antenna and a transmitter-receiver unit.

Frequently microwave systems transmit signals at one frequency and receive signals at a different frequency. Many systems of this nature were developed and installed prior to the availability of ferrite devices and consequently were unable to take advantage of the non-reciprocal circulation and absorption effects made possible by ferrite devices. Accordingly, such systems have often employed line length selection or other techniques in order to attempt to overcome problems introduced by internal reflections within the microwave system. A primary problem in this type of system is caused by refiections of transmitted energy from the antenna back into the transmitter. When there is extensive reflection, the transmitter oscillator is subject to frequency pulling and signal distortion. As a result a limitation is placed on performance bot'n as to the number of channels which can be accommodated and the signal-to-noise ratio of the existing channels.

This situation is merely one example of a need which is often encountered for greater isolation between antenna and transmitter in a microwave system. While nonreciprocal isolators can be used for this purpose, it is often difficult for these devices to operate with a sufficiently low insertion loss at the high transmitted powers which may be involved. Additionally, there may not be sufficient isolation afforded by this arrangement if it is designed for a sufficiently low level of insertion loss. An important factor is always found in the size and weight of systems of this type, and in this respect the length of isolator section and magnet which are required may be too great to permit their use with standard designs. An important consideration is that whatever coupling device is to be used must be readily insertable into existing equipment without requiring more than a short length of line, or much additional space.

It is therefore an obiect of the present invention to provide an improved means for affording greater isolation between antenna and transmitter in a microwave system.

Another object of the present invention is to provide an improved system, directly insertable in existing systems, for providing effective isolation between the transmitter and antenna of a microwave system with low insertion loss.

A further object of the present invention is to provide an improved coupler affording effective isolation cornbined with low insertion loss between the transmitter and antenna of a microwave system while providing low insertion loss for received microwave energy at a different frequency.

Systems in accordance with the invention provide marked increases in isolation between the transmitter and antenna of a microwave system by utilizing a combination of circulation and band filtering techniques. A small-size and lightweight circulator, filter and load are all that need be inserted between a transmitter and antenna in order to provide a marked improvement in signal-tonoise ratio, and to eliminate the adverse effects of reflections from the antenna.

rates Patent A specific example of the system in accordance with the invention utilizes a ferrite T circulator coupled between a transmitter-receiver and an antenna of a dual frequency system. One arm, the input arm, of the circulator is coupled to the transmitter-receiver, and arranged to circulate transmitted signals to a second or output arm, which is coupled to the antenna. Transmitted signals which are reflected from the antenna back into the circulator are circulated to the third arm, to which is coupled a bandpass filter which accepts signals at the transmitted frequency. A non-reflective load connected to the filter absorbs this reflected microwave energy. Received signals intercepted by the antenna are also circulated to the filter, but are refiected to the arm which is coupled to the transmitter-receiver and no losses are introduced.

In accordance with one aspect of the invention, a ferrite T circulator of small size and short length is employed, so that the unit may be added directly to existing microwave systems, to act as a single frequency load isolator. This ferrite T circulator is relatively broadband while the filter is relatively narrow band. As a result, the circulator will accept microwave energy at both the transmit and the receive frequency, while the filter will only absorb the reflected transmit energy and will transmit, without loss, the received energy.

A better understanding of the invention may be had by reference to the following description taken in conjunction with the accompany drawing, in which:

FIG. l is a schematic block diagram of a microwave signal system including a coupling arrangement in accordance with the invention; and

FIG. 2 is a perspective view, partially broken away, of wave guide elements forming the coupling system of the invention.

As shown in the generalized representation of FIG. l, a microwave signal coupling system 10 in accordance with the invention may be mounted between a transmitterreceiver system f2 and an antenna system 14 which may include a splash plate 1S. The transmitter-receiver 12 is of the type which transmits microwave energy on one frequency and receives microwave signals on another, somewhat related, frequency. Thus it may comprise a microwave radar or communication system having a single output terminal for both transmitting and receiving. It may operate as a transponder unit, or as a beacon unit. In any event the important consideration is to be able to couple the signals to be transmitted from the transmitterreceiver l2 to the antenna i4 efficiently, without high VSWR (voltage-standing-wave-ratio), and in such a way that energy reflected from the antenna 14 back toward the transmitter-receiver l2 does not adversely affect the operation of the transmitter-receiver l2 during the transmit mode by pulling or distorting the transmitter oscillator. Furthermore, it is just as important that the receive mode not be adversely affected.

The microwave signal coupling system includes principally a relatively broadband circulator device 15, a relatively narrow band filter i7 and an absorber i8. These elements employed in this combination, when used and disposed in accordance with the present inven* tion, achieve the above mentioned objectives.

An example of a specific form of microwave signal coupling system useful in these combinations is shown in the detailed perspective, broken away, representation of FIG. 2. The circulator 16 is advantageously a ferrite T circulator constructed from a rectangular waveguide, and using a centrally disposed ferrite element or post 20 within the T waveguide section 23. The ferrite post 2i) is a rod of circular cross-section as shown. With this configuration, the -arm 21 which is coupled to the transmitter-receiver is labeled the input arm, and

the arm 22 which is coupled to the antenna 14 is labeled the output arm. It should first be noted that with this configuration the spacing between the external ends of the input and output arms 21, 22 may be extremely small, an in practical installations may be of the order of three inches, including the fian-ges coupled to the ends of the arms. As shown generally in FIG. 1, the entire microwave signal coupling system may be constructed as a unit, and encapsulated or potted by a suitable plastic potting compound within an Iouter envelope, for physical integrity and resistance to environmental effects.

In accordance with conventional T circulator operation, the ferrite post is magnetized in a direction along the length of the electric field vector in the waveguide section 23. A generally C-shaped magnet 24 external to the waveguide section 23 has its pole faces disposed opposite the ends of the ferrite post 26. For achieving better dircctionality while minimizing reiiection losses, the ferrite post 20 is separated from one face of the magnet 24 by a rectangular .pad 25 which is of non-magnetic material such as brass, and which may have a slight circular recess to receive the end of ferrite post 20.

A number of expedients which are employed in the magnetizing arrangement contribute advantageously to the superior results such as the broadbanding of the circulator which are achieved. Magnetic field strength and distribution in the region of ferrite post 20 may be enhanced by the use of a magnetic shielding arrangement in conjunction with the magnet 24. It will be understood that two pole pieces (not shown) may also be employed if desired. The magnet 24 is encompassed on three sides by non-magnetic shielding members including two side plates 27, 28 and a connecting plate 29 lforming a housing for the magnet. Each of these plates 27-29 may be formed from several separate thin sheets of a non-magnetic material such as brass or aluminum.

The colinear input and youtput arms 21, 22 of the waveguide section 23 terminate in conventional fianges which are coupled to the associated devices. The shunt arm 31 of the circulator 16 also terminates in a conventional iiange, shown schematically, which is coupled to to the filter section 17 of the system.

The filter section 17 and termination section 18 are advantageously constructed in a compact, single unit which has the desired electrical properties. The filter section 17 is formed of a rectangular waveguide 30 as a cavity-coupled, three-section, filter. Each of the three cavities is defined by irises 32-35 extending inwardly from the side walls of the waveguide and positioned at different points along the length of the waveguide 30. The successive discontinuities introduced by the irises 32-35 provide the equivalent of inductive elements, and define three separate chambers 36, 37 and 38. Of these chambers, chamber 36 defined by the first and second irises 32, 33 and chamber 38 `defined by the third and fourth irises 34, 35 provide resonant chambers which are relatively wide. The intermediate chamber or cavity 37 defined by the second and third irises 33, 34 is relatively narrow and acts as a coupler between the two chambers 36, 38 on either side. The resonant frequency of each of the chambers 36, 38 may be adjusted by tuning screws 40, 41 set in bosses 42, 43 and arranged approximately midway on the broa-d wall of the waveguide 30 and about -midway of the chambers 36, 3S so that the tuning screws 40, 41 may be entered a controlled amount within the respective waveguide chambers to effect a capacitive adjustment. Further fine tuning adjustments may be made by other tuning screws 44, 45 and 46 also set approximately midwidth into the same broad wall of the rectangular waveguide 30. The tuning screws 45 and 46 are approximately in line with irises 32 and 33 respectively. Once the screws have been adjusted in a conventional manner for the proper frequency of the filter Section 17, they may be fixed into position by an lappropriate sealant material.

This arrangement of the filter section 17 permits a very close or exact adjustment of the two chambers 36, 38 to a selected resonant frequency. However, the presence of the cavity coupler 37 develops a second resonant frequency from the component reflections from the four irises 32-35, this frequency being adjacent to that for which the two chambers 36, 38 were individually adjusted. In accordance with known techniques, the bandpass of the filter section 17 may be broadened or narrowed within limits in accordance with the frequency bands which are to be handled.

The termination section 18 utilized with this coupler consists of a high-loss dielectric element 50 mounted directly in the open end of the rectangular waveguide Sil. Refiections at the interface of the dielectric 50 and the waveguide 30 are minimized by employment of a quarter wave step at the edge of the dielectric element 50 which is adjacent the filter section 17. A wide variety of well-known materials are useful as the absorber formed by the dielectric element 50. v

Devices are known for providing isolating functions, and many systems are also known and employed for diplexing operations. Diplexers are, of course, coupling systems which permit operation at separate frequencies with the same antenna. The usual methods of isolation and diplexing, however, cannot be performed together without some loss in the characteristics of one or both. Thus Where an isolator may be placed in a section of transmission line to attenuate energy reflected from the antenna, it will also usually markedly increase the VSWR and the insertion loss for received signals. Diplexing systems usually involve the use of a number of junctions arranged in a particular Way, and therefore are large in size, heavy and not suitable for addition to existing installations. Couplers in accordance with the invention, however, such as the specific coupler shown in FIG. 2, operate to eliminate the frequency pulling problem caused by reflections of transmitted signals from the antenna. On the other hand, they afford low insertion loss for received microwave energy.

The ferrite T circulator 16 is both a physically compact and electrically efficient mechanism for providing coupling between the desired terminals. The ferrite element 20 is magnetized in a direction along its length by the encompassing magnet 24. What may be called the input signals from the transmitter-receiver 12 are coupled directly from input arm 21 through the ferrite loaded section to the output arm Z2, for application to the antenna 14. This coupling is effected by what is generally regarded as a field displacement phenomenon, with little insertion loss in the direction from input 21 to output 22, but with isolation in the reverse direction. Thus signals at the transmitter frequency are coupled directly to the antenna 14. Signals which are reiiected at the antenna 14 are not returned directly to the transmitter-receiver 12, however, but are diverted into the shunt arm 31 of the T waveguide section 23.

The T waveguide section 23 which is ferrite loaded provides a compactI and lightweight type of 3-port circulator. Usually, however, the use of such a device is limited to a single nominal frequency, because `of narrow-band characteristics. In the present application, however, signals which are applied to the T waveguide section 21 from the output side 22 are also diverted to the shunt arm 31. l ust as is the case with signals at the transmitter frequency, signals at the receiver frequency have little insertion loss, and good isolation in the reverse direction.

This relatively broadband characteristic for the T waveguide section 23 is achieved in part by the employment of the impedance matching pad 25, and further in part in an integrated manner by the presence of Ithe magnetic shielding formed by the side plates 27, 28 and the connecting plate 29. For these reasons, signals from the direction of the output side 22 are all directed by arm 31 toward the filter section 17.

By adjustment of the tuning screws 40, 41 and the fine tuning screws 44, 45 and 46, the filter section 17 appears substantially as a lossless transmission section to signals at the transmitter frequency, and substantially as a reflector to signals at the receiver frequency. The signals at the transmitter frequency are therefore absorbed in the dielectric element 50 within the absorber section 18, constituting a dummy load.

rThe coaction between the elements of the ferrite-loaded T waveguide section 23 and the filter section 17 is an important characteristic of systems in accordance with the present invention. Without both excellent directivity of the signals applied to the T waveguide section 23 and the relatively broadband characteristics of section 23, and eX- cellent frequency selectivity charactertistics in the filter section 17, an appreciable part of the reflected, transmitted energy from the antenna 14 might be returned to the transmitter side, while at the same time appreciable losses in received energy might be encountered. Actually, however, the total insertion loss for both the transmit and receive directions, is less than one db. The isolation to actual antenna refiection is greater than 30 db in such practical installations. With a filter section 17 as shown in FIG. 2, a fine tuning of the reject response may be achieved in which a filter has approximately a 16 megacycle bandwidth, and is approximately 20 db down across the entire band.

A practical device in accordance with the present invention employs the following dimensions for the principal elements, and has the operating relationships given below:

Frequency: C band; transmitter and receiver 100 mc. separated in the region of 6700 mc.

Waveguide: RG-SO/U .750 X 1.500 nominal outer dimensions.

Circulator section: 2.500" long (including fianges).

Ferrite: Cylindrical .430" long X .370" dia.

Pad: 1.110 x 1.218 X .200.

Filter section: 4.905" long, including absorber filled end.

Irises: .710 X .540" X .031".

Length of cavities: .930, .550, .930.

Absorber section: 1.109 overall length.

Absorber first step: .207 (high) X .610" (long).

Absorber second step: .613 X .349.

Absorber material: Carbonyl E of Radio Cores Co.,

Forest Park, Ill.

While there has been described above and illustrated in the drawings a particular form of coupling arrangement in accordance with the invention, it will be appreciated that the invention is not limited thereto. Accordingly, the invention should be considered to include all variations and alternative forms falling within the scope of the appended claims.

What is claimed is:

1. In a microwave system having a transmitter-receiver and an antenna, and transmitting microwave energy at a frequency different from that of the received energy, a unitary mechanism for coupling between the transmitterreceiver and the antenna and for eliminating the adverse effects of refiections of transmitted signals from the antenna, including the combination of a ferrite T circulator, said ferrite T circulator having one arm coupled to the transmitter-receiver and a second arm coupled to the antenna, the direction of circulation being such that transmitted energy from the transmitter-receiver is coupled to the antenna, a bandpass filter coupled to the remaining arm of said circulator, said bandpass filter having a pass band characteristic to accept signals at the transmitted frequency and to reject signals at the received frequency, said circulator being coupled to said bandpass filter so as to circulate signals at the received frequency to the transmitterreceiver, and load absorbing means coupled to said filter to dissipate signals accepted thereby.

2. In a microwave system having a transmitter-receiver and an antenna, and transmitting microwave energy at one frequency and receiving microwave energy at a second, different frequency, a unitary mechanism for coupling between the transmitter-receiver and the antenna and for eliminating the undesired effects of reflections of transmitted signals from the antenna back to the transmitter-receiver, including the combination of a ferrite T circulator, said ferrite T circulator having a first arm, a second arm and a third arm, said ferrite T circulator being relatively boardband so as to pass microwave energy from said first arm to said second arm and microwave energy at both the transmitted and received frequency from the second arm to the third arm, and to pass microwave energy from said third arm to said first arm, said first arm being coupled to the transmitter-receiver and said second arm coupled to the antenna, a relatively narrow band bandpass filter coupled to said third arm of said circulator, said bandpass filter having a passband characteristic so as to accept signals at the transmitted frequency and to reject signals at the received frequency, and load absorbing means coupled to said filter to dissipate signals accepted thereby.

3. A microwave system including a transmitter` receiver having a common terminal, said transmitterreceiver providing transmitted microwave signals at a first frequency and being arranged to accept received microwave signals at a second frequency, an antenna for both transmitting and receiving, and an isolating and diplexing unit including, in combination, a ferrite T circulator having three arms, and a centrally disposed internal ferrite element, magnetic means for biasing said ferrite element to provide circulation of input waves from a first arm to a second arm, input Waves from the second arm to a third arm, and input waves from the third arm to the first arm, the first arm being coupled to the transmitter-receiver, the second arm being coupled to the antenna, a relatively narrow-band, quarter-wave, resonant filter coupled to the third arm of the ferrite T circulator, said filter being arranged to accept the transmitted frequency, and provide a high level of rejection of signals at the received frequency, and a dummy load coupled to said filter to absorb signals accepted thereby.

4. The invention as set forth in claim 3, wherein said ferrite T circulator is magnetically shielded, and wherein said dummy load includes successive impedance matching sections.

5. In a microwave system including a transmitterreceiver having a common terminal, the transmitterreceiver providing transmitted microwave signals at a first frequency and being arranged to accept received microwave signals at a second frequency, an antenna for both transmitting and receiving; an isolating and dipleX- ing unit including, in combination, a ferrite T circulator having three arms, a centrally disposed internal ferrite element, magnetic means biasing said ferrite element to provide circulation of input waves from a first arm to a second arm, input Waves from the second arm to a third arm, and input Waves from the third arm to the first arm, means for magnetically shielding said circulator, the first arm being adapted to be coupled to the transmitter-receiver, the second arm being adapted to be coupled to the antenna, a relatively narrow-band, quarter- Wave, resonant filter coupled to the third arm of said ferrite T circulator, said filter having a passband arranged to accept the transmitted frequency, and provide a high level of rejection of signals at the received frequency, and a dummy load coupled to the free end of said filter to absorb signals accepted by said filter.

6. In a microwave system having a transmitter-receiver and an antenna, and transmitting microwave energy at a frequency different from that of the received energy, a unitary mechanism for coupling between the transmitter-rcceiver and the antenna and for eliminating the adverse effects of reflections of transmitted signals from the antenna, including the combination of a three-terminal ferrite circulator having one arm coupled to the ytransmitter-receiver and a second arm coupled to the antenna, the direction of circulation being such that transmitted energy from the transmitter-receiver is coupled to the antenna, a bandpass filter coupled to the remaining arm of said circulator, said bandpass filter having a pass band characteristic to accept signals at the transmitted frequency and to reject signals at the received frequency, said circulator being coupled to said bandpass filter so References Cited by the Examiner UNITED STATES PATENTS 3/1962 salzberg 3337-73 12/1962 Wheeler 333-24 X 10 ELI LIEBERMAN, Primary Examiner.

HERMAN K. SAALBACH, Examiner. 

1. IN A MICROWAVE SYSTEM HAVING A TRANSMITTER-RECEIVER AND AN ANTENNA, AND TRANSMITTING MICROWAVE ENERGY AT A FREQUENCY DIFFERENT FROM THAT OF THE RECEIVED ENERGY, A UNITARY MECHANISM FOR COUPLING BETWEEN THE TRANSMITTERRECEIVER AND THE ANTENNA AND FOR ELIMINATING THE ADVERSE EFFECTS OF REFLECTIONS OF TRANSMITTED SIGNALS FROM THE ANTENNA, INCLUDING THE COMBINATION OF A FERRITE T CIRCULATOR, SAID FERRITE T CIRCULATOR HAVING ONE ARM COUPLED TO THE TRANSMITTER-RECEIVER AND A SECOND ARM COUPLED TO THE ANTENNA, THE DIRECTION OF CIRCULATION BEING SUCH THAT TRANSMITTED ENERGY FROM THE TRANSMITTER-RECEIVER IS COUPLED TO THE ANTENNA, A BANDPASS FILTER COUPLED TO THE REMAINING ARM OF SAID CIRCULATOR, SAID BANDPASS FILTER HAVING A PASS BAND CHARACTERISTIC TO ACCEPT SIGNALS AT THE TRANSMITTED FREQUENCY AND TO REJECT SIGNALS AT THE RECEIVED FREQUENCY, SAID CIRCULATOR BEING COUPLED TO SAID BANDPASS FILTER SO AS TO CIRCULATE SIGNALS AT THE RECEIVED FREQUENCY TO THE TRANSMITTERRECEIVER, AND LOAD ABSORBING MEANS COUPLED TO SAID FILTER TO DISSIPITATE SIGNALS ACCEPTED THEREBY. 